In the conventional ventilator for a coldroom with pressure responsive valves, a partition wall is provided within a ventilation path adapted to interconnect the inside and the outside of the coldroom, and valve ports bored in the partition wall are opened and closed by means of valve plates which come into contact with or are free from the partition wall in response to an increase or reduction of internal pressure. However, such ventilator has encountered such a problem that there occur buildups of frost and ice due to a large difference in temperature within and without the coldroom. When such buildups of frost and ice exceed a certain limit, the valve plates become fixed to the partition wall and hence will not become free therefrom.